1. Field of the Invention
The present invention generally relates to the art of processing electrical signals. In particular, the present invention relates to the art of noise cancellation in analog circuits.
2. Description of Related Art
Many integrated circuit (IC) devices include both digital circuits and analog circuits. Such an IC is referred to as a mixed-signal integrated circuit (MSIC). The application of the MSIC is most prevalent for IC's used in the multimedia and communications devices such as wireless communication systems, or cellular telephones. Even in mostly digital circuit IC's, analog circuits are used to achieve functions not realizable or difficult to implement using purely digital circuits.
Even though the use of analog circuits may be advantageous in many designs, the IC industry has been attempting to minimize the use of analog circuits for IC chips having digital circuits because of the susceptibility of the analog circuits to noise generated by the digital circuits. Digital circuits, especially the larger digital circuits prevalent in the industry, are very noisy relative to typical analog circuits surrounding the digital circuit. The analog circuits may fail due to the noise generated by the digital circuits.
Moreover, increasing miniaturization of electronic devices, especially in the communications market, has required the IC chips to become even more tightly integrated. Consequently, the circuits comprising the IC chips, both digital and analog, are being fabricated close to each other, thereby aggravating the negative effects of the noise.
Previous attempts to alleviate the problem focused on the method of shielding circuit blocks of an IC or focused on isolating the circuits of the IC. For example, the U.S. Pat. No. 4,628,343, entitled “Semiconductor Integrated Circuit Device Free From Mutual Interference Between Circuit Blocks Formed Therein,” issued to Yuji Komatsu, discloses an IC where “the first and second circuit blocks are shielded electrically from each other on the surface of the semiconductor chip.” Col. 2 II. 27-30, the Komatsu reference. In the U.S. Pat. No. 5,453,713, entitled “Noise-Free Islands in Digital Integrated Circuits,” issued to Hamid Partovi and Andrew J. Barber, the “integrated circuit chip has both digital and analog circuit functions, with one or more islands for isolating the analog functions from noise caused by the digital functions.” Abstract, the Partovi and Barber reference. However, in tightly integrated, compact IC packages, shielding or isolation techniques may not be desirable, sufficient, or even feasible.